Answer
The speed of the object at $s=20m$ is $7.07m/s$
Work Step by Step
The net work done by the force is the area of the red shape as portrayed in the force-displacement graph. $$\sum W=\frac{1}{2}(10m+20m)\times10N=150J$$
The work-energy theorem states that $$\sum W=\frac{1}{2}m(v_f^2-v_0^2)$$
We know $v_0=0$ and $m=6kg$. Therefore, $$3v_f^2=150$$ $$v_f=7.07m/s$$ which is the final speed of the object at $s=20m$
